Phallus-Shaped Creature Is Wormy Missing Link

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A fossilized creature shaped (let's just say it) remarkably like
a penis may be the missing link connecting two mysterious
branches of sea creatures.

The fossils, more than 9,000 specimens in all, reveal a wormlike
animal with an "elongate posterior trunk ending in a bulbous
unit," as researchers describe it in this week's issue of the
journal Nature. The animal appears to be a transition in the
evolution of wormlike tube feeders known as
pterobranches.

Pterobranches are part of a group called the hemichordates, along
with another bunch of wormy sea creatures called enteropneusts,
or acorn worms. But while pterobranches are tiny and stay in one
place, filter-feeding from
colonies of tubes on the seafloor, solitary acorn worms move
about in burrows, feeding on organic material that drifts down to
the ocean floor. Acorn worms range in size from a few millimeters
to a few meters long.

An overlooked link

The links between these two groups are mysterious, but now
scientists from the Royal Ontario Museum, the University of
Cambridge and the University of Montreal say they may have found
the connection in the Burgess shale. This
formation in the Canadian Rockies holds fossils from the middle
Cambrian Period, about 505 million years ago.

Previously, the oldest acorn worms, or enteropneusts, dated back
about 300 million years, said study researcher Jean-Bernard
Caron, the curator of invertebrate paleontology at the Royal
Ontario Museum in Toronto. But the 505-million-year-old Burgess
shale held enteropneusts much older than that.

In fact, the specimen, now named Spartobranchus tenuis,
is one of the most common fossils found in the Burgess shale,
Caron told LiveScience. Smithsonian Institution paleontologist
Charles Walcott first reported the discovery of the worms in
1911. [ See
Photos of the Phallic Worm ]

"He just wrote three lines about this worm," Caron said. "He was
not sure what it was. And basically nothing was done since."

A wormy ancestor

Caron and his colleagues analyzed 9,000 specimens of the worm —
no easy task, Caron said, because in many cases the portions of
the fossils containing key anatomical features were missing or
covered up by the bodies of other fossilized worms.

The creature grew to about 4 inches (10 centimeters) in length
and had a phallic body shape (not unlike modern acorn worms)
lined with gills. But most important, it is found fossilized with
a tube structure about 25 percent of the time, much like a modern
pterobranch.

"They are like the enteropneust worms, but they live in tubes,
which are quite branching and quite rigid," Caron said. "We think
it is from a tube of this kind that the pterobranch tubes
evolved."

The finding clears up a mystery about whether today's modern
hemichordates started out as tube-dwelling worms or as mobile
burrowers.

"For once, the fossil record has spoken in a voice that is more
or less unambiguous," Nature editor Henry Gee wrote in a
commentary accompanying the study.

The discovery also pulls back the curtain on the origin of the
chordates, a group of animals with spinal cords that includes
vertebrates such as humans, Caron said. Hemichordates, chordates
and echinoderms like starfish and urchins all had a
common ancestor with gill slits, Caron said.

"We think it's possible that the common ancestor for all three
groups was wormlike," Caron said.